1. Field of the Invention
This invention relates to electronic theft detection systems (also known as electronic article surveillance apparatus); and in particular it concerns improvements for enabling such systems to interrogate and detect articles marked with targets which resonate at different frequencies.
2. Description of the Prior Art
Various techniques have been used to detect shoplifting or unauthorized removal of articles from protected areas. One of the most successful techniques, which is disclosed in now expired U.S. Pat. No. 3,500,373, involves affixing resonant circuit targets to the protected articles, generating a swept radio frequency interrogation field in the region of an exit from the protected area and detecting the occurrence of predetermined disturbances to the field caused by the passage of a resonant circuit target through the interrogation field.
As the electronic article surveillance industry has developed, different systems have been supplied which operate at different frequencies. At the present time, most resonant frequency type electronic theft detection systems operate either to detect resonant circuit targets which resonate at 2 MHZ (megahertz) or to detect resonant circuit targets which resonate at 8 MHZ. However, the 2 MHZ system cannot detect targets which resonate at 8 MHZ and the 8 MHZ system cannot detect targets which resonate at 2 MHZ. Consequently, once a proprietor of a store invests in one type of system he cannot change over to the other type unless he is willing to substitute his entire inventory of resonant circuit targets.
It has been proposed to provide separate detection systems which operate at 2 MHZ and 8 MHZ respectively. However, in order to avoid mutual interference the systems must be placed a substantial distance from each other; and the exit passageway from the store must be designed to require patrons first to pass between antenna panels of one system and thereafter to pass between antenna panels of the other system. This arrangement causes much wasted space and is inconvenient for patrons. It has also been proposed to place the two systems adjacent each other and operate them in a time sharing sequence. This proposal causes problems because the mere proximity of the transmitter antennas of the two systems produces a mutual coupling which adversely affects the interrogation signals. Further, in situations where the systems are installed along adjacent exit passageways, the systems are already time shared in order to separate the signals produced in the different passageways. Further time sharing to separate the signals produced at different frequencies would greatly reduce the durathon in which a given target is monitored and this increases the risk that it will escape detection. On the other hand, if the systems are not time shared, their respective frequency sweeps will interact and cause intermodulation components. This raises the background noise level incident on the higher frequency system; and in some cases it produces signals which are similar to those produced by a target being carried past the antenna panels. Consequently, there is a danger that the system will produce false alarms.